Consumption of energy-containing added sugars (AS) and in particular, sugar-sweetened beverages (SSB), have been suggested as contributors to weight gain. In children and adolescents, total AS intake represents ~16% of total energy, or ~300-400 kcal/d; SSB comprise ~50% of total AS intake. Although recognized by major health organizations, the role of AS and their primary food source, SSB, in the development and progression of obesity and related co-morbidities remains controversial. A common research limitation in this area is a reliance on self-reported measures of dietary intake, which present additional challenges when studying children. Thus, the need for objective methods to assess dietary intake, such as biomarkers of AS consumption, has been recognized. We have established the validity of the fingerstick blood d13C AS biomarker in adults, and aim to expand our innovative biomarker to studies of diet in children. We propose to establish the validity and reliability of the fingerstick blood d13C AS biomarker in children using two approaches. First, a controlled feeding component (Study 1) will provide data necessary for validation of the biomarker with actual AS intake, and determine its ability to detect levels of AS intake. Second, a cross-sectional component (Study 2) will compare the biomarker to self-reported intake data, collected in a method similar to national nutritional surveillance methodology (i.e., NHANES). Study 1 will include 30 adolescents aged 12-18 yrs, who will consume both a high AS (25% total energy) and low AS (5% total energy) diet for 7 days each, in a random order. Study 2 will include 325 children aged 6-18 yrs, who will complete five laboratory sessions. Record- assisted 24-hr dietary recalls will be completed at four of the sessions to assess habitual AS intake, and fingerstick blood samples will be obtained at two of the sessions. The potential confounding effects of non- sweetener corn and animal product consumption will be addressed in both studies by quantifying non- sweetener corn consumption in the controlled diets (Study 1) and in self-reported dietary recalls (Study 2), and by assessing the nitrogen stable isotope composition 15N of fingerstick samples. To advance existing knowledge of dietary assessment approaches, urinary sugars and urine d13C will be assessed in Study 1, which will permit a direct comparison of biomarkers - existing (urinary sucrose, fructose) and novel (urine and fingerstick d13C ). The role of AS in health has been contentious for decades, and the reliance on self-reported intake data is an often-cited flaw in this area. Our findings could significantly advance research addressing the health impacts of AS intake in children and adolescents.